Many Next-Generation Sequencing (NGS) platforms require asymmetrically-tagged fragments, i.e., DNA fragments that are flanked by different adaptor sequences. The adapter sequences are necessary for clonal amplification on the flow cell surface, and may also be used as priming sites for insert or barcode or sample index sequences. For example, in Illumina's system, the fragments have the P5 sequence on one end and the P7 sequence on the other. Such molecules are not straightforward to produce efficiently, largely because most of the methods that are used to add adaptor sequences to fragments do so in a random way. If the adaptor sequences are added in a random way, only 50% of the tagged fragments are asymmetrically tagged with the correct sequences (e.g., the fragments have P5 at one end and P7 at the other); the other 50% of the tagged sequences are symmetrically tagged (e.g., the fragments have P5 at both ends or P7 at both ends). Only the asymmetrically fragments are suitable for sequencing on many sequencing platforms.
When sequencing the genome of a single cell or a small number of cells, the 50% loss of coverage caused by the randomness of adaptor attachment is a serious problem. For this reason, new strategies need to be developed.